This invention relates to a microfiltration (MF) or ultrafiltration (UF) membrane of organic polymer for filtering a desired liquid from a liquid mass referred to herein as a "substrate".
The membrane may be in the form of a capillary tube or hollow fiber membrane (or "fiber" for brevity), or, in the form of a tubular sheath of film formed either on the inner or outer surface of a tubular macroporous support; or a laminar sheet or film; or a laminar sheet or film deposited on a porous support. In the art, a hollow fiber of microporous polymer, or, a tube of braid having a nominal inside diameter of less than 2.5 mm, coated with a semipermeable film of microporous polymer is referred to as a hollow fiber membrane. A hollow fiber membrane which does not require a support, by definition, is self-supporting. A tubular sheath of thin film of polymer, by itself, or a sheet of thin film, by itself, is non-self-supporting and must be supported. The term "membrane" is used to refer to a film or sheet or the hollow fiber membrane in its entirety, irrespective of the form in which it is deployed. A particular example of a hollow fiber membrane is an extruded hollow fiber membrane with an outer diameter in the range from about 0.25 mm to 2.5 mm and a wall thickness in the range from about 0.15 mm to 1 mm, typically being in the range from about 5% to about 40% of the outside diameter of the fiber. The average pore cross sectional diameter in a fiber may vary widely. For MF, the average pore diameter is in the range from about 0.08 .mu.m to about 2.0 .mu.m, preferably from about 0.1-1 .mu.m. For UF, the average pore diameter is preferably in the range from about 0.01 .mu.m to 0.1 .mu.m. An example of a supported membrane is a flexible laminar sheet; or a tube of knitted or woven flexible braid coated with the tubular film, the tube having an outside diameter in the range from about 0.5 mm to about 5 mm. For the sake of clarity, reference to the film, by itself, is made with the term "film membrane", or "thin film" or "film" for brevity, since without the film there would be no membrane. Since the support for a film membrane has macropores which are very large relative to pores within the film, they are referred to herein as "voids".
A tubular sheath of non-supporting film has such a thin wall, in the range from 0.01 mm to 0.09 mm thick, that the tube will collapse unless supported by fluid. If a thin sheet of film 0.09 mm thick is either extruded or cast, a piece of the film in a small square 10 cm on each side, has so little strength that, by itself, it cannot be manually or mechanically manipulated without being damaged. Because of its very thin cross-section and non-self-supporting nature, such a film, derived from the synthetic resinous material provides a semipermeable film having excellent semipermeable properties so long as the film is suitably deployed, and, a geometry favored by the film, is maintained. The membrane may be operated as MF or UF under a vacuum drawn on the "lumens" (bores of the fibers) in the range from 1 mm (0.02 psi) to about 517 mm (10 psi) of Hg, and under an overall differential in hydrostatic pressure in the range from about atmospheric 101 kPa (14.7 psi) to 300 kPa (43.54 psi), preferably less than 275 kPa (40 psi) for MF flow; and, from about 300 kPa (43.54 psi) to about 690 kPa (100 psi), preferably less than 600 kPa (87 psi) for UF flow.
The art of forming either self-supporting hollow fibers, or a non-supporting thin film supported on a tubular braid is well known, given the specific polymer which has been found to lend itself to being formed with the physical structure required to function as a semipermeable membrane for filtration of a liquid.
Though numerous ultrafiltration membranes are available, the search to find a membrane with optimum properties is unremitting. The problem is to find a membrane which allows filtration of the desired liquid with a high flux which is maintained over a long period of operation. Knowing that a polymer can yield a semipermeable membrane having a gradient porosity therethrough, with requisite orientation, is insufficient information for one skilled in the art to make the membrane. For example, polyvinylidene difluoride (PVDF) in a specified range of molecular weights will yield a filtration membrane, however its long term performance will be poor due to fouling, and, to filter an aqueous substrate with desirable performance, should be made hydrophilic. Polypropylene will yield a filtration membrane only if it is oriented after it is cast in a particular range of thickness, and should be prepped, for example with alcohol, before using the membrane to filter an aqueous substrate. Knowing that polymer when cast or extruded will yield a microfiltration or ultrafiltration membrane, one skilled in the art must still know details relating to how the solution of polymer ("dope") is to be manipulated, if they are to prepare a membrane which is usable for a specified purpose.
The physical solution to the problem lay in finding a particular, highly stable polymer which lent itself to having its structure modified so as to produce a membrane with excellent flux and reliability in operation.
U.S. Pat. No. 5,130,342 to McAllister et al discloses particle-filled microporous materials in which substantially non-agglomerated inert filler particles are dispersed in a thermoplastic polymer. There is no evidence provided that a usable filtration membrane can be formed as disclosed, and no evidence that any membrane formed as disclosed was effective as a filter in any liquid substrate.
Japanese JP 58093734 A teaches that a hydrophilic PVDF membrane is produced by treating a membrane of PVDF, or copolymers of PVDF, containing a fine powder of hydrophilic inorganic particles, with an aqueous alkali. Silicic acid, calcium silicate, alumina and magnesium oxide are specified, and it is stated that use of these powders enables the wetting treatment of the membrane to be omitted. There is no suggestion that any specified powder or contents of the powder, which would react with the polymer and calcined .alpha.-alumina is not specified. Neutral alumina will not react with PVDF in a solvent in which neutral alumina is inert, and such particles substituted for calcined .alpha.-alumina particles are relatively ineffective. It is well known to treat a PVDF membrane (not PVDF polymer), first with base, then with acid, to improve its hydrophilicity. Preparing the complex, as we have, avoids the post-treatment of the membrane.